1. Field of the Invention
The present invention relates to a high efficiency, low emissions fired heater unit utilizing positive pressure with a vertically arranged convection section. The use of positive pressure permits forced convective heat transfer which reduces the necessary size for heating, which results in a compact furnace design. It is contemplated that the fired heater unit in accordance with the present invention will be used in refinery and petro-chemical applications.
2. Discussion of Related Art
In petroleum refineries and petro-chemical plants, fired heaters are typically employed because these heaters can provide a higher level of heat, which typically can not be obtained from other utility sources. The conventional fired heaters presently in commercial practice contain a radiant section, a convection section, an air preheater and ultra low NOx burners. The convection section is located on top of the radiant section. The convection tubes of the convection section have a horizontal orientation.
The conventional fired heaters utilize preheated air to maximize heater efficiency. The conventional fired heaters require a balanced draft design with a forced draft blower and an induced draft blower. The forced draft blower feeds the ambient air to the preheater and then to the heater. The induced draft blower withdraws the flue gas. This arrangement, however, is prone to the ingress of additional ambient air. The ingress of ambient air has a negative impact on the operation of the heaters. As a result, the conventional fired heater must be operated at 3% or higher excess O2 levels to prevent combustion related instabilities. This reduces heater efficiency such that only a typical maximum achievable heater efficiency of is 91-92% is attained. Reductions in efficiency result in increases in total NOx and greenhouse gas emissions.
With an ever increasing demand on fuel, its rising cost, and at the same time tighter regulatory control on emissions reduction, there is a need for a fired heater that maximizes heater efficiency while reducing emissions. This need especially exists in larger heaters (greater than 100 MBtu/hr).